Method of electropolishing molybdenum

ABSTRACT

A METHOD OF ELECTROPOLISHING WORK PIECES AS MOLYBDENUM ELECTRODES AND GRIDS AND REMOVAL OF OXIDATION ON STAINLESS STEEL, WITH A CURRENT DENSITY ON THE ORDER OF 0.9 TO 5 AMPERES PER SQ. INCH IN A VOLTAGE RANGE IN THE ORDER OF 15 TO 90 VOLTS, USING AN ANHYDROUS ACETIC ACID BASED BATH CONTAINING ABOUT 10 PERCENT SULFURIC ACID. THE PROCESS IS RELATIVELY FREE OF VARIOUS HAZARDS COMMON TO THE ELECTROPOLISHING OF METALS, E.G. HIGH CELL VOLTAGE, EXCESSIVE METAL LOSS AND DANGEROUS ELECTROLYTE COMPOSITIONS.

United States Patent vs. Cl. 204-1405 4 Claims ABSTRACT OF THE DISCLOSURE A method of electropolishing work pieces as molybdenum electrodes and grids and removal of oxidation on stainless steel, with a current density on the order of 0.9 to 5 amperes per sq. inch in a voltage range in the order of 15 to 90 volts, using an anhydrous acetic acid based bath containing about 10.percent sulfuric acid. The process is relatively free of various hazards common to the electropolishing of metals, e.g. high cell voltage, excessive metal loss and dangerous electrolyte compositions.

-This application is a continuation-in-part of Ser. No. 721,153 and now abandoned.

BACKGROUNDOF THE INVENTION The polishing of metals mechanically and electrotlytically has been known 'for' many years; Brimi, M.A., Electrofinishing American Elseiver Publishing Co. Inc. (1965); Cortes, F. R. Electrolytic Polishing of Refractory Metals, Metal Progress 80, No. 2, 97-100 (1961) and Tegart, W. 1., The Electrolytic and Chemical Polishing of Metals in Research and Industry, Pergamon Press (1959).

In the electrochemical field combination of hydrofluoric and other acids, create mechanical and physical hazards and are characterized by high cell voltage and current derisity coupled with excessive metal loss. This has created a desire for less hazardous treating compositions in conjunction with improved control of metal loss, particularly in the elfecting smoothness and polishing of molyb denum electrodes and grids or molybdenum helixii, and deoxidizing stainless steel. 7

SUMMARY OF THE INVENTION This invention relates to an improvement in a method of deoxidizing stainless steel and electropolishing molybdenum metal work pieces, and especially molybdenum and molybdenum alloy anodes and grids, compositions therefor, and the products thereof. More particularly, the invention relates to.animprovement in the electrolytic method of deoxidizing stainless steel or electropolishing of molybdenum electrodes and grids, more economically convenient compositions therefor and the products thereof.

Accordingly, it 'is an object of this invention to provide the art with an improvement overcoming the above problems and the improved metal articles produced thereby. o

It is a more particular object of this invention to provide a more economicaland convenient method for improving the electropolishing and improving electrical characteristics of molybdenum electrodes and-grids or helices, providing compositions therefor and the products thereof.

It is a further object of this invention to provide an improvement in the deoxidizing of other metal particularly objects of stainless steel.

objects and 3,679,559 Patented July 25, 1972 DESCRIPTION OF THIS INVENTION ILLUSTRATED BY PREFERRED EMBODIMENTS In the electronic field, the use of molybdenum electrodes in vacuum tubes has appeared to be advantageous, but the application of high voltage thereto has caused corona to occur with subsequent loss of tube operation.

However, it was discovered that a smooth clean surface having better electrical properties, and principally reduction of high voltage corona on molybdenum electrodes and molybdenum-rhenium grids or helixii, was obtained with a non-aqueous solution of the following bath composition:

parts glacial acetic acid 10 parts concentrated sulfuric acid or about 10 ml. concentrated sulfuric acid to about ml. glacial acetic acid, as the active polishing components.

The metal work piece (a molybdenum electrode), treated in the above composition, was first thoroughly cleaned with a vapor degreaser, or Freon, or both. It is imperative that the parts to be deoxidized or polished be clean and free of oil and wax. Repeated cooling and dipping into a solvent therefor as trichloroethylene vapors etfectively removes the oil and wax.

The cleaned metal piece, as the molybdenum electrode, was then positioned in the bath, as the anode. The molybdenum electrode is preferably supported by molybdenum clips or rods to avoid contaminants. Otherwise, stainless steel, copper, brass, and the like support rod or clip means may be used with the appropriate metal and preferably masked with a conventional dip-type maskant. The small molybdenum electrode (as the anode), positioned facing the cathode, was electropolished with a 1 ampere current flow at 17 volts in 3 minutes time. Under these conditions, the rate of metal removal for the surface being polished is 0.05 mil/minute. This rate of removal, in the relative time, is much smaller than for any other baths known to the art. Small electrodes are polished without stirring, but mild agitation of the large anodes appears, to be necessary and helps obtain uniform polish. In the present processing, a current density of 0.9 amp per sq'. in. is being utilized with the voltage pushed up to 90 volts. With this increased voltage at a current density, on the order of 0.9 amp. per sq. in., the molybdenum metal loss is not observably or materially changed. Thus, in the overall the current density may controllably range from about 0.9 ampere to about 5 ampere per sq. in. with the voltage correspondingly regulated and controlled between the order of 15 to 90 volts. For best results'the lower current density is utilized with the higher voltagev and the greater current density with the lower voltage, as the operators choice for best results with the metal work piece being treated in the solution.

After polishing and rinsing in preparation for use, the resultant electrode product was discovered to have improved electrical properties, as notably reduced high voltage corona. Also, such polishing prevented arcing between high voltage elements and grounded element s, thus enabling greater than normal voltage to be impressed on such treated electrodes.

Other tests with this solution illustrated that low current densities and higher voltages or higher current density and low voltage yielded excellent polishes in a few minutes with metal removal being very small in a few minutes process. Also, the solution did not heat up to any excessive temperature. It is also useful for polishing inside a small hole, and obtaining a much smoother polish than possible by hand.

While some variations may be possible in solution concentrations, temperature and time, the polishing function perature unless etching results are desired in hot solutions.

Excellent polishing is obtained with the anhydrous bath at room temperature and there is practically no temperature build-up in the bath during the polishing operation. With the acetic acid bath, no cooling is required. In

continuous operation, the temperature raise may be slight to about 30 C. This may be overcome by cooling, for con-' tinuous operation, if desired. The voltage range required is only 17 to 30 volts up to the limit of 40 volts, or lower voltage on the grids. However, it is now discovered that theyoltage can be pushed up to about 90 volts with a current density 'of 0.9 amp. without further observable or appreciable loss of molybdenum in the anode work piece. Below 12 volts, little polishing occurred, if any. At 15 volts, excellent polishes were obtained up'to'the limit of 40 volts on grids and up to 90 volts on the anodes. At higher current values and voltages, the rate of metal removal and etching of molybdenum becomes excessive. Herein, excellent finishes are provided with metal removal of from 0.1 mil to 0.15 mil in 3 minutes in the bath with low current densities and of about 40 to 90 volts.

' The voltage and current are coupled and dependent on the cell resistance. The bath employed requires about 1 ampere/6 cm. (1 ampere/in). As indicated in recent development, it has been discovered that rapid polishing can be accomplished by controlling the amperes and raising the voltage, as indicated. During the polishing or deoxidizing process, the current normally reaches a maximum after a few seconds, and then drops to a steady value. i

Molybdenum electrodes generally do not require more than 3-4 minutes polishing time in the method herein provided. At the volt-ampere/in. setting of about 20,"

the bath will remove 0.05 mil per minute. For small focus electrodes, the voltage should be about 17 and the cur rent will stabilize at about 1.5 amperes. For large anodes, 30 volts should be used and the current will be 4-5 amperes at completion of the time period. As recently V discovered, the voltage maybe raised to 90 volts and with moved and cleaned. Rinsing with tap water, distilled water, and dipping in an alkaline bath, as ammonium hydroxide, maybe used. After the final rinse (preferably distilled water) .the polished metal is preferably rinsed. withmethanol, dried and packaged, or ready for use.

As indicated, the anode is placed in the ,bath facing the cathode and each work piece generally dictates its ownconfiguration of the cathode. For example, if large or deep holes are present, they should 'be equipped with a separate, or auxiliary, cathode in the center, and large anodes may require a ring cathode placed centrally and in close proximity to the surface of the work piece. 7

As described, the anhydrous composition of sulfuric and acetic acid is preferred for the electropolishing of molybdenum electrodesat relatively low current densities and low voltage of not over about 90 volts with improved control of metal removal, without obvious or detrimental pitting and in the order of 0.1 mil for about a threeminute process. 1

For'uniform and improved results, the polishing operation is better effected in clean bath compositions and treating baths'from which an excess of sludge has been removed by replacing an equivalent amount of new solution for the removed sludge portionQ H I In general, in the anhydrous composition described of essentially acetic acid and sulfuric acid, the

action requires the formation ofa viscous filrnon the surface of the work piece being polished and this depends primarily on the solution viscosity. Other metals, for ex-- h reinample, aluminum requires agitation for proper electropolishing, copper requires no agitation and, in fact, agitation leads to striations and uneven surface finish. When the anhydrous acetic acid bath is firstprepared and a metal work piece immersed, the nature 'of the viscous layer may be readily observed. It is colored blue and drips off the work piece or flows in a laminar fashion at about the speed of free-flowing molasses. Agitation, effecting removal of the viscous layer, causes etching and severe' striation formation. Consequently, no bath agitation per se is used with the acetic-,sulfuric-acid bath, provided It "has been discovered, however, that mildagita'tion' of the work piece is beneficial on large (3%" diameter) parts. Without such movement thereof, etching occurs. This etching appears to be related to the thickness of the viscous layer and the low current which accompanies it. Smaller samples howeverdo not require movement cause the viscous layer thins itself out fast enough to effect proper current flow. This is, on a large work area, the viscous layer cannot be dissipated so that the current falls below an optimum value and etching occurs without agitation of the work piece. p

-It has thus been discovered that there is herewith provided an electropolishing bath of an anhydrous acid combination for polishing a metal' surface and particularly molybdenum anodes were accomplished in 3-5 minutes polishing time at low current densities of about 1.5 to 4-5 amperes and low voltage of 15 to 40 volts and pref erably in the order of 17 to 30 volts. Wonk samples polished in 3-4 minutes, as small molybdenum focus electrodes, were previously maintained at a'voltage of about 17 and the current stabilized at about 1.5amperes. .For larger anodes, 30 volts were used and the currentwas stabilized at 45 amperes at the completion of the timed period. In generalization, for reproducible polishing, the volt-amperes/square inch should be in the region of 20 with the higher ampere range. In the-lowerampere range. of about 0.9, the voltage can be pushed up to about for comparable results. For example, as indicated, the"- small focuselectrodes have ay.-a./in. of about 17 and the large anode has a value of about 23 for the bath to remove about.0.05 mil from theworkpiece surface a are suspended in the solution and treated therein in a like manner.

electrode orv tableware, when suspended and treated in a like manner, as the molybdenum workpiece, is deoxidized without any observable or readily determinable loss of metal. The particular current density andvoltage utilized with respect to time is under control of the operator and results desired. c

Having described the present embodiments of our discovery in accordance with the patent statues, it will nowbe apparent that some modifications and variations may be made without departing from the spirit and scope thereof. The specific embodiments described are providedby way of illustration and are illustrative of our discovery, invention or improvements which are to be limited only by the termsof the appended claims.

What is claimed is: c

1. The method of polishing a molybdenum metal workpiece surfacecomprising the steps of:

active polishing constituents in the order of parts acetic acid to 10 parts concentrated sulfuric acid; 4. The method of claim .1 wherein the workpiece is (b) supporting a molybdenum workpiece to be polagitated in said bath.

ished as an anode workpiece in said bath; (c) positioning a cathode in facing relationship to the R f e Ci surface of the said molybdenum workpiece. 5

(d) establishing a controlled voltage on the order of UNITED STATE? PATENTS '15 to 90 volts and a controlled current on the order 1,334,092 3/ 1920 Hafmellng 204145 of about 0.9 ampere to 5 amperes on said molyb- 2,315,568 4/ 1943 wqfllllmd 204145 denum workpiece; and 3, 02,899 10/1961 Re1d 204-441 (c) efiecting a polishing of said molybdenum by re- 10 FOREIGN PATENTS moval of metal therefrom in the order of .05 mil per minute and in the order of 1.5 mils in a time 576591 5/1959 Canada 204-1405 period of 3 to 5 minutes. l 2. The method of claim 1 wherein the voltage is in HOWARD WILLIAMS Primary Examiner the order of 20 to 90 volts. 15 T. TUFARIELLO, Assistant Examiner 3. The method of claim 1 wherein the workpiece is a small molybdenum electrode supported in a stationery position during polishing. 204-445 R 

